While not limited to that application, the invention is particularly useful in connection with the taking of photographic pictures of a compass, or inclinometer, or both, at the bottom of a well bore. Well drillers can control the direction of deep wells by control of the drilling tools. But control is accomplished in terms of adjustment to change from current direction as drilling proceeds. That means that the driller must know the wells' current direction from time to time. Current direction is determined by lowering a compass inclinometer to the bottom of the well and then photographing the compass assembly to record its direction in azimuth and inclination. The task of photographing a compass at the bottom of a well is both complicated and expensive.
The compass needle is acted on by the earth's magnetic field. To permit that, the compass must be disposed in a non-magnetic section of pipe at the time that the photograph is taken. Current practice is to include a short length of non-magnetic pipe at or near the lower end of the drill pipe. That length of pipe is ordinarily made of Monel and it is called a "collar." The compass must be disposed in that section when it is photographed. Care must be taken to ensure that the compass assembly is not in motion at the time that the photograph is taken. Current practice is to include a detector which detects absence of motion and a means for precluding exposure of the film for some selected time interval measured from the last action which could cause movement of the compass needle.
To prevent premature exposure, it has been necessary to ensure that instrument motion does not stop until the instrument has reached the position at which the photograph is to be taken. To overcome that requirement, a conventional clock has been used so that photograph taking occurs at some fixed time after the clock is started at the well head. That solution is subject to failure if, for any reasonn the time required to lower the instrument is other than the predicted time.
This invention relates to the problem of ensuring that the compass is in the non-magnetic collar and to the problem of postponing picture taking until the compass needle has "settled down" before the film is exposed. To lower the apparatus into, and to retrieve if from, a deep well is very costly. It is important to be able to determine reliably and with a high degree of certainty whether the compass is, or is not, in the non-magnetic section of the drill pipe.